\documentclass{scrreprt}
\usepackage[english]{babel}
\usepackage{geometry,blindtext}
\geometry{inner=2.5cm, outer=5cm}
\geometry{includehead, includefoot, top=2.5cm, bottom=2cm}

\begin{document}
\blindtext[2]

%ab hier kleinere Ränder, mehr bedruckbare Fläche.

\begin{addmargin}[-1cm]{-3cm}
\blindtext[4]

\end{addmargin}

\end{document}